Elaborate all type of keys in sql?

Question

Sunil Singh · Answer

Let’s take an example of a Student table and understand the concept of keys in the database in more clear way:

CREATE TABLE Student (    StudentID, 
    StudentName, 
    DOB, -- Date of birth
    Subjects
    PMN, -- Personal mobile Number
    DeptID, -- Department ID
    class
)

1. Candidate Key: is the attribute/column or a set of attributes/columns in a relation/table that qualifies for uniqueness of each tuple/row. A relation/table can have one or more than one Candidate Keys. A Candidate key is also known as a minimal Super key.

Here in Student table columns StudentID & PMN individually can maintain uniqueness in a table, thus are eligible for Candidate keys. The columns StudentName + DOB combined can also make up a Candidate Key, but there is a narrow chance that 2 Students with same name can be born in same day.

2. Primary Key: is the Candidate key attribute/column that is most suited to maintain uniqueness in a table at the tuple/row level.

Here in Student table you can choose either StudentID or PMN column for a Primary key, StudentID is preferable choice because PMN is a secure (PII) value contains privacy.

3. Alternate Key: are the other Candidate key attribute/columns that you didn’t choose as Primary key column.

Like if you choose StudentID as a Primary key then PMN would be the Alternate key.

4. Super Key: It is a superset of Candidate key. If you add any other attribute/column to a Candidate Key then it became a Super Key.

Like StudentID + StudentName is a Super Key.

5. Composite Key: If a table do have a single column that qualifies for a Candidate key, then you have to select 2 or more columns to make a row unique.

Like if there is no StudentID or PMN columns in Student table, then you can make StudentName + DOB as a Composite Primary Key. But still there can be a narrow chance of duplicate rows.

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Elaborate all type of keys in sql?

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